You have to borrow spare hard disks worth the total amount of your data. Transfer it over. Then put your existing drives into NAS (4bay or bigger) and rebuild Raid6 (Yes go for Raid6 with 2 hard disks for redundancy.) New Raid6 building will be a boring process which can take 1-2 days.

Once your setup is ready, transfer over the files which you backed up. Return the borrowed drives to your friends.

Also avoid NAS like the plague, If its a big array and you suspect youll be adding more disks to it then a home made rack server is the way to go. Just get a rack case like a Norco and a P800 RAID card or something similar (can be picked up for 100 bucks with 2xBBU online)

Don't make the same mistake I did using SATA, 20 SATA cables = Major PITA you cab get SAS > 4x SATA cables